Datasheet

Monitoring Voltages Other than V
CC
Monitor voltages other than the V
CC
by connecting a
voltage divider to PFI and adjusting the ratio appropri-
ately. If required, add hysteresis by connecting a resis-
tor (with a value approximately 10-times the sum of the
two resistors in the potential divider network) between
PFI and PFO. A capacitor between PFI and GND will
reduce the power-fail circuit’s sensitivity to high-fre-
quency noise on the line being monitored. RESET
can
be asserted on other voltages in addition to the +5V
V
CC
line. Connect PFO to MR to initiate a reset when
PFI drops below 2.50V (K, L, N suffix) or 1.70V (T suffix
or MAX816). Figure 11 shows the MAX814K/L/N/
MAX815K/L/N configured to assert RESET when the
+5V supply falls below the reset threshold, or when the
+12V supply falls below approximately 11V.
Monitoring a Negative Voltage
The power-fail comparator can also monitor a negative
supply rail (Figure 12). When the negative rail is good
(a negative voltage of large magnitude), PFO is low.
When the negative rail is degraded (a negative voltage
of lesser magnitude), PFO is high. By adding the resis-
tors and transistor as shown, a high PFO triggers reset.
As long as PFO
remains high, the MAX814/MAX815/
MAX816 will keep reset asserted (RESET
= low, RESET
= high). Note that this circuit’s accuracy depends on the
PFI threshold tolerance, the V
CC
line, and the resistor.
Watchdog Software Considerations
A way to help the watchdog timer keep closer tabs on
software execution involves setting and resetting the
watchdog input at different points in the program,
rather than pulsing the watchdog input high-low-high or
low-high-low. This technique avoids a stuck loop where
the watchdog timer continues to be reset within the
loop, keeping the watchdog from timing out. Figure 13
shows an example flow diagram where the I/O driving
the watchdog input is set low at the beginning of the
program, set high at the beginning of every subroutine,
then set low at the end of every subroutine. If the pro-
gram should hang in any subroutine, the I/O is continu-
ally set high and the watchdog timer is allowed to time
out, causing a reset to be issued.
MAX814/MAX815/MAX816
±1% Accuracy, Low-Power, +3V and +5V
µP Supervisory Circuits
______________________________________________________________________________________ 13
MAX814
MAX815
MAX816
RESET
R1
MAX814K/L/N
MAX815K/L/N
RESET
100kΩ
1%
PARAMETER
MIN TYP
MAX UNIT
+12V Reset
Threshold at +25°C
10.57
11.00
11.45 V
340kΩ
1%
+5V
+12V
V
CC
PFI
GND
MR
PFO
TO µP
Figure 10. RESET Valid to Ground Circuit Figure 11. Monitoring Both +5V and +12V